ABSTRACT
Rapid urban development and population growth in Palm Beach County, Florida,
have been accompanied with the need for additional freshwater withdrawals from
the surficial aquifer system. To maintain water quality, County officials
protect capture areas and determine zones of transport of municipal supply
wells. A multistep process was used to help automate the delineation of
wellhead protection areas.

A modular ground-water flow model (MODFLOW) Telescopic Mesh Refinement
program (MODTMR) was used to construct an embedded flow model and combined
with particle tracking to delineate zones of transport to supply wells; model
output was coupled with a geographic information system. An embedded flow
MODFLOW model was constructed using input and output file data from a
preexisting three-dimensional, calibrated model of the surficial aquifer
system. Three graphical user interfaces for use with the geographic
information software, ArcView, were developed to enhance the telescopic
mesh refinement process. These interfaces include AvMODTMR for use with
MODTMR; AvHDRD to build MODFLOW river and drain input files from dynamically
segmented linear (canals) data sets; and AvWELL Refiner, an interface designed
to examine and convert well coverage spatial data layers to a MODFLOW Well
package input file. MODPATH (the U.S. Geological Survey particle-tracking
postprocessing program) and MODTOOLS (the set of U.S. Geological Survey
computer programs to translate MODFLOW and MODPATH output to a geographic
information system) were used to map zones of transport.

A steady-state, five-layer model of the Boca Raton area was created using the
telescopic mesh refinement process and calibrated to average conditions during
January 1989 to June 1990. A sensitivity analysis of various model parameters
indicates that the model is most sensitive to changes in recharge rates,
hydraulic conductivity for layer 1, and leakance for layers 3 and 4 (Biscayne
aquifer).

Recharge (58 percent); river (canal) leakance (29 percent); and inflow through
the northern, western, and southern prescribed flux model boundaries
(10 percent) represent the major inflow components. Principal outflow
components in the Boca Raton well field area include well discharge
(56 percent), river (canal) leakance (27 percent), and water that discharges
along the coast (10 percent).

A particle-tracking analysis using MODPATH was conducted to better understand
well-field ground-water flow patterns and time of travel. MODTOOLS was used to
construct zones-of-transport spatial data for municipal supply wells. Porosity
estimates were uniformly increased to study the effect of porosity on zones of
transport. Where porosity was increased, the size of the zones of transport
were shown to decrease.